Internal heating for comfort of residential and commercial dwellings is very often practised by heating and circulating water in carbonaceous-fuel-fired or electric heaters and passing the hot water through convection heat exchangers (often called radiators) to heat the inside surrounding air while cooling the water. The cooled water is then returned to the fired heater to be reheated. Usually the water is heated to as high as 80° C. and the return water is cooled to near ambient room temperature as low as 20° C. so as much as 60 kcal/kg of sensible heat is taken from and needs to be re-supplied to the water by the combustion of a carbonaceous fuel such as natural gas or by the consumption of electric power.
In the combustion of, for example natural gas, carbon dioxide (CO2) and water vapour are formed into the flue gas while roughly 10,000 kcal/kg of combustion heat energy is released to heat water. In efficient modern heating systems, the exiting flue gas is partially cooled by heat exchange with the inlet cool water to within about a 20° C. approach to the inlet temperature giving a rather high thermal efficiency; sometimes with partial combustion water condensation. The flue gas is then admitted to a stack in a chimney at around 50° C. and very often saturated with water vapour. Emission of this humid flue gas heat to the atmosphere afflicts an appreciable loss of a potential humid heat energy which could be recovered. Today the improvement of the energy efficiency, especially, for carbonaceous-fuel-fired systems is a very important concern for the residential or commercial dwelling owner not only to reduce the expense of fuel energy consumption but also to reduce the emission of the greenhouse gas CO2 known to impact on global warming.
When the latent heat of water condensation, roughly 550 kcal/kg of water, can be beneficially captured to useful heating from combustion of methane, for example, there would immediately be an increase in thermal efficiency resulting in more than 10% savings in the fuel required since one kg of methane combusted yields slightly more than two kg of water.
Many improvements have been made today to the conventional carbonaceous fuel fired heating system. Some incorporate heat exchange equipment to condense combustion water. However, low temperatures and high flow rates of cold fluids are needed to condense an appreciable amount of water from flue gases. Lest it be for preheating an enormous flow rate of cold air, these low heat exchange temperatures are generally not suitable for transfer of heat to a circulating water system used to warm a residential or commercial dwelling during the colder months of the year. Condensing beat exchange is not suitable for fuels heavier than LPG where more excess air is needed resulting in flue gas water dew point temperatures which are considerably lower. Moreover, water condensation heat exchange surface requires a resistance to carbonic acid corrosion which results from the presence of CO2 in the flue gases. The resulting condensed sour water may not be simply passed to a closed drain for disposal and may require some treatment. If sulphur impurities would also be present in the carbonaceous fuels, water condensation should be completely avoided due to even more severe sulphurous acid type corrosion and condensed water pollution.
Inside the average residential dwelling there are many other sources of humid energy.
Some are generated simply by the breathing of human beings and animals living therein. Some are generated from activities related to drying laundry and cooking (here boiling water to cook potatoes is a good example). Most of the time these sources of humidity are ventilated to the outside to enhance the internal comfort level. Capture of this humid energy could be very beneficial if it can be achieved in a cost effective and useful manner. Dwellings heated with electricity where combustion water is not formed could also benefit from the collection of these other humid heat sources to save electric power consumption. Commercial dwellings also contain such sources of humid energy, for example the breathing of workers and the boiling of kettles.